1. Field of the Invention
The present invention relates to noise reduction circuit architecture, more particularly, to providing a noise reduction circuit architecture for communications applications.
2. Related Art
Typically, wind, air conditioning, and busy traffic introduce significant noise energy at frequencies below 150 Hz, compared with the energy levels of human voices over the bandwidth 300 Hz to 3,400 Hz. This type of low frequency ambient noise and/or wind turbulence noise, commonly referred to as wind noise, has posed special problems in communications applications.
For example, in the case of a portable headset microphone, wind noise amplitude can be very large, compared with the speech levels. A strong wind noise has a power level approximately 10 dB to 30 dB higher than the power level of a typical human voice. Wind noise generally has a frequency less than 1 kHz, and the lower the frequency, the higher the noise power.
Based on the sound sensing characteristic of the human ears, the lower frequency noise reduces one's ability to discern sounds at frequencies above the noise frequencies if the low frequency noise power is significantly higher than the voice power. Accordingly, the dynamic range of an audio codec front end diminishes with the amplitude of the wind noise.
One conventional means of solving this problem is through the use of a dedicated dynamic high-pass-filter. In such a solution, a detector determines the noise intensity and adaptively moves the high pass filter poles in response to the level of the noise intensity. Such a dynamic high pass filter is conventionally realized on a chip that is separate from the subsequent amplification and digital processing capabilities. However, such an implementation severely distorts the sound characteristic. When the wind noise is strong, the adaptive process will cause the poles of the dynamic filter to fall within the audio band. For example, when the noise intensity is high, the pole frequency will potentially be set higher than 1 kHz. As a consequence, the low frequency content of the desired audio is compressed, which in turn reduces voice intelligibility and sound fidelity.
The sound fidelity issue can be overcome by another conventional solution, namely the use of a brick-wall high pass filter. As the name suggests, a brick-wall high pass filter maintains a flat response across the entire audio frequency band. In order to realize such a flat filter response, the high pass filter must be of a very high order. This in turn demands large capacitance values and significant silicon utilization. However, such a silicon requirement is too big to be practical for consumer electronics applications.
A conventional alternative to a filtering approach to the wind noise program is to use a programmable gain amplifier (PGA). In response to the presence of strong wind noise, the gain of the PGA is reduced in order to avoid clipping at the input to the subsequent analog-to-digital converter (ADC). However, there are a number of disadvantages with this approach. Firstly, the circuitry itself contributes a significant amount of noise. With this architecture, the input-referred noise contributed by the amplification stage inside the PGA increases as the PGA gain is reduced. The effective noise generated in later stages also increases when the overall PGA gain is reduced. In addition, as the overall PGA gain reduces to accommodate the strong wind noise, the available full scale signal range also reduces. Furthermore, to avoid signal attenuation from the external microphone bias network, the input resistance of the PGA has to exceed a minimum threshold. Such a minimum limitation places a further limitation on the ability of the high pass filter formed by the input resistance and the AC coupling capacitance to effectively reduce the effects of the wind noise.
What is needed is a new noise reduction circuit architecture that provides improved low frequency noise reduction and sufficient audio fidelity while minimizing the need for additional components in a voice communication system.